Electrochemical cells having a lithium anode have become increasingly important to battery technology as the search for additional sources of energy continue. Lithium, due to its extremely high electrochemical potential, can and often times does find itself paired with a depolarizer or cathode material in an electrochemical cell to produce potentially superior cells and batteries.
With the evolution of the lithium battery has come the desire to employ liquid depolarizers. These liquid materials have a high degree of mobility, thereby offering the potential of complete utilization of that material. In addition, the open circuit voltage between soluble or liquid depolarizers and lithium is extremely high. Thus, interest has focused on what appears to be a potential source of a relatively high amount of energy at a relatively high potential.
While a number of potential candidates have been proposed as depolarizers for use with lithium, several materials, notably sulfur dioxide and thionyl chloride have become the subject of substantial development due to their many advantageous properties. U.S. Pat. No. 3,578,500 discloses sulfur dioxide batteries in which one or more of the cosolvent potentially available to that cell is thionyl chloride. U.S. Pat. No. 3,891,457 teaches the use of thionyl chloride itself as a depolarizer in combination with certain cathode collector designs. In addition, U.S. Pat. No. 3,891,458 teaches the use of thionyl chloride as a cathodic depolarizer as well as a solvent in a system. The patent further discloses that in high rate applications, electrolytes using pure thionyl chloride as a solvent fail to yield high coulombic efficiencies when combined with lithium anodes. Increased cell capacities and efficiencies are disclosed employing the first solute or salt comprising a salt of the selected anode metal in combination with a second solute of a compound selected from the group consisting of phosphoryl chloride, sulfolane, sulfur dioxide and mixtures thereof. Improvements in cell capacity over a basic single salt solute cell having approximately 2.46 ampere hours are disclosed wherein up to 4.1 ampere hours of capacity are achieved. In each of these cases, a substantial quantity of the second solute is added which appears to materially increase the capacity of the cell.
As efforts continue to bring cells from the laboratory to the market place, however, certain difficulties have been incurred which have prevented lithium-thionyl chloride cells from reaching a significant number of commercial applications. This is particularly true in primary active batteries, which are, of course, those cells which are used once and are ready to discharge current from their time of manufacture. Unfortunately, few cells are taken directly from the assembly line to the device for which they are intended. The marketing process alone is normally responsible for long periods of delay between the initial manufacture of the active cell and its ultimate use. Moreover, cells are not purchased merely when they are needed, but, conventionally, are purchased well in advance of their need. Thus, the "shelf life" of a cell becomes a prime factor in its utility as an electrochemical primary active cell.
One of the problems that occurs with lithium-thionyl chloride cells is the build up of byproducts of the lithium anode. Specifically, during storage of lithium metal in contact with thionyl chloride as is the case in primary active cells, a nonelectronically conductive film of lithium chloride grows on the lithium surface. This film-forming reaction occurs as part of the overall electrochemical reaction in a manner similar to that during discharge. Specifically, at the anode, the lithium metal ionizes to produce electrons. At the cathode, or the thionyl chloride, electrons combine with the thionyl chloride to form sulfur, sulfur dioxide and chlorine ions. The lithium ions and the chlorine ions form a lithium chloride film which is insoluble and prevents further attack of the lithium by the thionyl chloride. However, this nonconductive film prevents the cell from operating at its normal voltage during the initial stages of cell discharge. This growth of apparently amorphous lithium chloride on the lithium anode takes place over a period of time. The longer a cell is stored prior to use, the greater the build up and the more difficult it is to operate the cell. This difficulty in operating the cell is particularly true when the cell is employed in a system which requires a high current drain. Low loads in the order of less than 1 or 2 mA/sq. inch of electrode area are not as severely affected by long storage as are situations where a relatively high current drain is initially required during use of the cell. In some circumstances, the cell is completely useless after long storage whereas a fresh cell can readily supply the current desired.